Coated articles are known in the art for use in window application such as vehicle windshields, insulating glass (IG) window units, monolithic vehicle windows, and/or the like. It is known that in certain instances, it is desirable to heat treat (e.g., thermally temper, heat bend and/or heat strengthen) such coated articles for purposes of tempering, bending, or the like in certain example instances. For example, vehicle windshields are made up of a pair of glass substrates that are heat bent and laminated together via an adhesive layer (e.g., PVB based layer). One of the glass substrates may have a coating thereon which is subjected to the heat treatment along with the glass.
In certain situations, designers of coated articles often strive for a combination of high visible transmission, substantially neutral color, low emissivity (or emittance), and low sheet resistance (Rs). High visible transmission for example may permit coated articles to be more desirable in applications such as vehicle windshields or the like, whereas low-emissivity (low-E) and low sheet resistance characteristics permit such coated articles to block significant amounts of IR radiation so as to reduce for example undesirable heating of vehicle or building interiors.
In applications such as vehicle windshields requiring a visible transmission of at least 70%, or even at least 75%, the coated article must be bent as well as heat treated. The bending is often performed by allowing a coated glass article to sag while being heat treated in a known manner. The heat treatment of such coated articles typically requires the use of temperature(s) of at least 580 degrees C., more preferably of at least about 600 degrees C. and often at least 620 degrees C., for about 5-10 minutes or more.
Unfortunately, too much bending with heat treating in making windshields often causes such coated articles to suffer damage known as mottling. Mottling defects essentially result hum cracking of the coating. Mottling appears in a windshield during the bending process, and is observed after the bending process is complete. It is detected as an optical distortion, similar to a vertical “crack” in the windshield. Mottling defects in windshields are often roughly 5 mm long and roughly 100 μm wide, and several microns deep in certain example instances. With certain coated articles, mottling occurs when the coated article is heat bent to a significant extent. It is believed that the mottling damage results from high degrees of bending during heat treatment.
FIG. 1 is provided for the purpose of explaining the degree of bending of a coated glass article such as a vehicle windshield. FIG. 1 is a cross sectional view of a bent vehicle windshield. Parameter “x” in FIG. 1 represents the amount of bend in the windshield, and is known as a centerline convex value. The centerline convex value x is the distance between the apex of the interior surface of the windshield drawn straight down to a line (see the dotted line in FIG. 1) connecting the two ends of the windshield. This centerline convex value x is representative of the amount of bend (or depth of bend) in the windshield; the higher the value x, the higher the amount of bend in the windshield. FIG. 2 is a cross sectional view of a heat bent vehicle windshield (e.g., of FIG. 1, or an embodiment herein) which includes first and second heat bent glass substrates 1, 40 laminated to one another via a polymer inclusive layer (e.g., of or including PVB or any other suitable polymer inclusive material) 42, where the low-E (low emissivity) coating 30 is provided on one of the substrates. While many known windshields are capable of withstanding bends of about 18 mm (i.e., a centerline convex value x of about 18 mm), they often cannot withstand bending to a greater extent without suffering from fatal mottling damage.
Consider the following coated article with the below-listed layer stack, where the layers are listed in order from the glass substrate outwardly.
Glass SubstrateTiO2Si3N4ZnOAgNiCrOxSnO2Si3N4ZnOAgNiCrOxSnO2Si3N4
While the aforesaid coated article is heat treatable, it cannot withstand significant degrees of heat bending without suffering fatal mottling damage. For example, such a coated article suffers fatal mottling damage at centerline convex values x of about 22-23 mm or more (i.e., a bend about 22-23 mm or more deep).
It will be appreciated by those skilled in the art that there sometimes exists a need for a vehicle windshield which is bent to a significant extent (e.g., bent to a centerline convex value x of at least about 24 mm, sometimes at least about 26 mm, or at least 28 mm, or even at least 30 or 32 mm in certain situations). Unfortunately, the coated article discussed above cannot be used in such applications because it cannot withstand such high degrees of bending without suffering fatal mottling damage.
Additionally, consider the following coated article with the below-listed layer stack, where the layers are listed in order from the glass substrate outwardly (thicknesses in angstroms).
Glass Substrate
Si3N4186 ÅZnAlOx107 ÅAg107 ÅNiCrOx 30 ÅSnO2520 ÅSi3N4131 ÅZnAlOx119 ÅAg103 ÅNiCrOx 33 ÅSnO2120 ÅSi3N4320 Å
While the above coated article realizes satisfactory results in many instances, it too is subject to mottling in high bend (during HT) applications used in windshields. Again, it will be appreciated by those skilled in the art that there sometimes exists a need for a vehicle windshield which is bent to a significant extent (e.g., bent to a centerline convex value x of at least about 24 mm, sometimes at least about 26 mm, or at least 28 mm, or even at least 30 or 32 mm in certain situations). Unfortunately, the coated article discussed above cannot be used in such applications because it cannot withstand such high degrees of bending without suffering fatal mottling damage.
Moreover, in vehicle windshield or other applications such as in insulating glass (IG) window units, lengthy heat treatments at high temperatures tend to cause the aforesaid coated article to suffer significant drops in visible transmission, significant changes in certain color value(s), and significant increases in sheet resistance (Rs). Thus, there is room for improvement in one or more of these respects. Additionally, the aforesaid coated article is susceptible to scratching in certain instances, and is also sometimes characterized by high haze values following heat treatment in certain instances.
In view of the above, it will be apparent to those skilled in the art that there exists a need for coated articles which are capable of one or more of: (a) being bent to greater extents in applications such as vehicle windshields or the like, (b) being able to maintain acceptable optical characteristics when bent to such extents, (c) realizing improved or good thermal stability with regard to visible transmission, color, emissivity (or emittance), and/or sheet resistance (Rs); and/or (d) realizing good mechanical durability such as scratch resistance and reduced mottling. In certain example embodiments, it may be desired that one or more of these characteristics can be achieved.